In commonly assigned U.S. Pat. Nos. 4,933,780 and 4,977,419, there is disclosed a photographic film having a virtually transparent magnetic layer covering the non-emulsion side of the film (referred to as an MOF layer) and dedicated recording areas or tracks on the layer for recording information such as film type, film speed, film exposure information and information relevant to the processing and subsequent use (e.g., printing) of the exposed image frames on dedicated longitudinally extending tracks. The systems disclosed therein provide for recording of information during film manufacture, reading and/or recording of information on certain tracks during camera use, and for reading and/or recording of print related information during photofinishing using the MOF layer. The information may include voiced messages or sound associated with the photographed scene and may be recorded in digital or analog format on certain of the tracks. The specific camera recording tracks and recording heads disclosed in the '419 and '780 patents are disposed to record in tracks located along the longitudinal edges of the film and outside the MOF tracks in the image frame. The possibility of recording audio within certain tracks of the MOF layer within the image frame area is described in commonly assigned U.S. Pat. No. 5,276,472.
Reading and recording information on tracks of a magnetic coating or stripes of magnetic coatings in the image frame area on photographic film in a still camera require solutions to problems different than those encountered in other apparatus. The space limitations in a portable hand-held still camera, which necessarily must be as compact and light as possible to appeal to the average consumer, restrain the size and location of the magnetic record/reproduce head and the film drive system. In addition, the head-to-film recording and reading interface differs in major ways than that normally associated with digital magnetic tape drives or even disk drives, as described generally in the above-referenced '780 patent. In the image frame area, scratches of the emulsion layers or the MOF layer and film base must be avoided to maintain the quality of the image.
In a tape drive, the magnetic tape is flexible enough under tension to conform to the head contour both in the tape feed direction and in the cross-tape direction. In a disk drive, the head flies or floats on the air film that is created as the flat surface of the disk rotates at high speed.
Magnetic tape is quite thin and exhibits virtually no stiffness. Photographic film is of much greater thickness than the magnetic tape used for commercial and consumer recording and reproduction. When removed from its cartridge, a filmstrip shows a relatively high stiffness and very observable cross-curvature across its width that is convex on the non-emulsion side of the film. Further, the unwrapped filmstrip also shows a convex curvature along its length, again on the non-emulsion side of the film. This latter curvature is attributed primarily to a core-set curl that results from the filmstrip having been tightly wound on a film cartridge spool.
The cross-curvature across the width of the filmstrip is primarily caused by the number of multilayers of emulsion and the MOF layer (if present). The emulsion multilayers (and the MOF layer, if present) have different stretch properties than that of the base film substrate of acetate, PET, or PEN material. The cross-curvature is also influencedby the bending phenomena known as anticlastic curvature. The degree of filmstrip cross-curvature also depends on environmental conditions, including the time and temperature history of the film, the relative humidity, and the thickness of the film. Because of the cross-curvature, it is difficult to achieve good contact or compliance across the width of a wide, multi-track recording head. To provide a reliable read or write signal, the magnetic head must remain in close proximity to the magnetic coating. Any disturbances, such as variations in film curl, can vary the relationship of the head to the magnetic coating and, therefore decrease the reliability of the signal.
When a photographic filmstrip 20 is wrapped around a cylindrical contour surface 52, as is typically used in a magnetic tape head 50, most of the cross-curvature described above is reduced. As shown in FIG. 1, the filmstrip 20 would normally be tensioned in a film plane of a film transport path by tension applied in the direction of arrows 54, 56. The contour surface 52 of head 50 penetrates into the film plane, and the filmstrip 20 is warped over the contour surface 52 defining a gap line 60.
Despite this penetration of the film plane, intimate compliance or contact of the filmstrip 20 surface to the cylindrical surface 52 is not achieved. The variability of spacing in microinches across the film at the gap line 60 of contact between the filmstrip and the cylindrical surface 52 is measured and plotted in FIG. 2. In this example, the head 50 is a cylindrical surface of radius 13 mm (0.523") and is modeled from glass for demonstration purposes. The cylindrical surface at the gap line 60 is penetrated into the film plane of filmstrip 20 at a wrap angle (W.A.) of 1.degree. and 3.degree., and a film tension of 55 grams is applied in each of the directions of the arrows 54, 56, FIG. 1.
A recording system specified for using the MOF layers requires that a spacing distance of 20 microinches or less along the gap line 60, that is, across the filmstrip MOF layer, be maintained to achieve specified record/reproduce performance. FIG. 2 shows that a spacing of greater than 20 microinches dominates over a vast portion of the interface at the apex of the compliance zone, with the exception of two narrow zones (zone 1 and zone 2). By changing the wrap angle to 3.degree. or more, the compliance zones remain concentrated approximately 1 mm inward from the filmstrip edges. As in the previous case, the compliance zones are narrow. However, the separation between them widens. The largest distance along the gap line 60, however, continues to have a spacing unacceptably greater than 20 microinches.
As shown in FIG. 3, the spacing distance along the gap line 60 modulates from 0 to near 500 microinches for an interface without emulsion side support (load=0). As expected, a considerable reduction in the distance spacing along the gap line 60 is seen with progressively larger emulsion side forces. The emulsion side support, composed of a low coefficient of friction material such as a nonwoven polyester fiber material supported on a compliant backing such as a silicone rubber, is used to apply a distributed pressure force ranging from 0 to 50 grams against the emulsion side of the filmstrip immediately opposite to the cylindrical head surface and aligned with the gap line 60. However, even at the pressure loads of 50 grams, that part of the gap line 60 between 5 mm and 8.7 mm has larger than acceptable spacing. In spite of the spacing improvement demonstrated, the use of a pressure pad for image area support is not favored in that potential scratching of the emulsion layers may result which diminishes photographic image quality.
Another problem unique to recording and reproducing information on or from MOF layers in photographic still cameras is that film advancement occurs in a short period of time with a limited amount of motion. Stepper drive motors are commonly employed to move the filmstrip an image frame at a time and do not provide the steady state media advance conditions which are normally associated with magnetic tape recording. Recording and playback must take place during transient conditions which tend to separate the film from the recording head. For optimum magnetic recording under these conditions, the magnetic head must maintain contact of less than 20, and preferably 10, micro-inches spacing with the MOF layer.
Techniques for maintaining the desired relationship of the head to a magnetic coating in other traditional apparatus, are not practical for use with such filmstrips in a photographic still camera, particularly a compact camera. For example, in a sound movie camera, a cinematic film having a magnetic stripe along one edge is typically continuously moved over a drum, and information is recorded by a magnetic head positioned in close proximity to the drum. The drum provides a rigid support for the film, removes film curl and assures a uniform head to film relationship. While a fixed support such as a sound drum produces satisfactory results, the space limitations in a photographic still camera render such a support impractical. Also, a drum/head interface is not suitable under the transient conditions described above.
Furthermore, in a still camera system, it is desirable to record information pertinent to and immediately coincident with the photographic images captured by the camera because negative filmstrips are sometimes cut up in photofinishing, and other reasons described in the above-referenced '780 and '419 patents. In sound movie cameras, recorded information is displaced from the image to achieve continuous motion of the film during recording as compared with intermittent motion during exposure.
The prior art relating to recording on photographic film thus generally teaches providing a pressure pad support for the recording medium on the side opposite from the recording head and continuous motion of the recording medium to ensure reliable recording. In U.S. Pat. No. 5,097,278, various configurations for mounting a magnetic head in relation to the film transport path and film plane with the use of an oppositely disposed pressure pad are disclosed in a camera. In the above-referenced '780 patent, a different approach is taken wherein the record/reproduce head of the camera is positioned with respect to the chamber for receiving a modified film cartridge. The magnetic head bears against the MOF layer of a filmstrip exposed through an opening in the film cartridge lip while the film is supported and flattened within the lip, the emulsion bearing against a compliant support formed with a light blocking plush material overlying a lower flange of the cartridge lip.
Yet another head interface problem in application of this technology in compact photographic still cameras is that film motion takes place in a frame stepping mode by means of a pulse of tension (i.e., jerking or jogging motion). When the pulse of tension is applied to a stationary filmstrip, the film tension conditions are highly variable. In a traditional tape recorder, this condition would lead to an unstable interface. One approach for controlling the film tension could be a continuously energized servo-control system. This seems impractical considering the large power drain on the camera batteries that would be required to maintain the film tension.